It's not uncommon to have an engine block that was cast the year prior to the model year of the car it is installed into. My 68 RT had a 67 block etc.

The assembly date for the engine I believe is stamped on the flat pad near the distributor opening. Obviously this should be a date after the casting date which is located on the side of the block.

On a 1968 block the VIN or partial VIN should be located near / behind the oil pressure sending unit port on the lip of the block (right where the transmission mates).

I'm not an expert and it's been a while since I had my 1968 R/T, so if I'm wrong about something, I'm sure someone will correct me.

Hope this helps.

-Dan